This invention relates to a timing signal regenerator for use in a modem of a difital communication network.
A digital communication network is operable in a predetermined one of an independent synchronization scheme and a dependent synchronization scheme. In a didital communication network of the dependent synchronization scheme, a timing signal regenerator is used in the modem of each slave station of the communication network. The modem is for modulating a carrier signal by transmission digital data into a transmission modulated signal for transmission to other modem or modems of the communication network and for demodulating a reception modulated signal received at the modem under consideration into reception digital data. The timing signal regenerator is for regenerating a reception timing signal from the reception modulated signal in phase synchronism with the reception modulated signal and for generating a transmission timing signal in frequency and phase synchronism with the reception timing signal. The reception timing signal is used in demodulating the reception modulated signal into the reception digital data. The transmission timing signal is used in modulating the carrier signal into the transmission modulated signal.
In the manner which will be described more in detail later in the following, a conventional timing signal regenerator comprises a reception timing signal generator for generating a first timing signal. A transmission timing signal generator generates a second timing signal. A phase error information extractor produces a first phase error information signal representative of first phase error information related to a first phase error which the first timing signal has relative to the reception modulated signal. The first phase error information signal is used in adjusting the first timing signal into the reception timing signal. A phase error information detector produces a second phase error information signal representative of second phase error information related to a second phase error which the second timing signal has relative to the reception timing signal. The second phase error information signal is used in adjusting the second timing signal into the transmission timing signal.
The transmission timing signal generator may comprise a voltage controlled oscillator. In this event, the second phase error information signal is used in controlling the voltage controlled oscillator to adjust the frequency of the second timing signal and thereby to adjust the phase of the second timing signal. As a consequence, it takes a long period of time in achieving the frequency and phase synchronism. Alternatively, the transmission timing signal generator may be operable in a phase locked loop. In this case, the second phase error information signal is discontinuously used in adjusting the frequency of the second timing signal to thereby attain the frequency and phase synchronism. As a result, jitter in inevitable in the transmission timing signal.